1) Field of the Invention
The present invention relates to an optical waveform measuring apparatus and an optical waveform measuring method, and more particularly to optical waveform measuring apparatus and method suitable for use in observing an optical pulse waveform, so as to make a faithful observation of an actual waveform. The optical pulse waveform encounters difficulty of accurate observations in fine detail through the use of electronic measurements, due to operating speed limitations on electronic circuits.
2) Description of the Related Art
In optical communications, the observation of an optical pulse waveform is a function needed for the evaluation of quality of an optical signal at receiver ends, and it is also applicable to a signal repeater or a node of an optical network for monitoring quality of signal. In addition, it exhibits a function indispensable to the evaluation of optical parts. Along with an increase in capacity for optical fiber communications, the bit rate of a signal light reaches 40 Gb/s at the research stage. Moreover, the research and development of a system exceeding 160 Gb/s have taken place very actively, aiming at a next-generation system. The promotion of these research and development requires a waveform measuring technique and waveform measuring apparatus capable of observing an actual waveform of signal light faithfully.
Now, in an optical waveform measuring apparatus commonly put to use, as shown in FIG. 23, an optical signal (optical pulse) under measurement is first converted through a photoelectric converter 101 into an electric signal. Following this, an electric sampling pulse is generated by an electric sampling pulse generating circuit 103 in accordance with an electric trigger from a trigger circuit 102 so that the electric signal from the photoelectric converter 101 is sampled with the electric sampling pulse in a sampling circuit 104 and the sampled waveform is displayed on a wave form indicator 105. That is, the wave form measurement is made by sampling an actual waveform of an optical signal with an electric signal.
At this time, for the faithful observation of the waveform of signal light, there is a need to receive all the frequency components constituting the signal light, which requires that the waveform measuring apparatus has a sufficiently broader bandwidth than the bit rate of the signal light. The observation bandwidth for the electric sampling measurements is limited by a device whose bandwidth is the narrowest among a photoelectric converter, a trigger circuit, a sampling circuit, a waveform indicator and other devices which configure the apparatus. Usually, an operating bandwidth limitation of an electronic circuit, considered to be 50 GHz, is not always sufficient to the fine observation of a signal of 40 Gb/s.
One solution to the problem on the bandwidth limitation involves an optical sampling technique of sampling signal light intact. According to this optical sampling technique, as shown in FIG. 24, an optical sampling gate 111 made of a nonlinear medium generates intensity correlation signal light relative to a light (wavelength: λsig) under measurement and a sampling light pulse (wavelength: λsam) shorter in pulse width than the light under measurement, thus outputting a sample result. In this case, the intensity correlation signal light signifies light generated when a light under measurement and a sampling light pulse overlaps with each other in the time domain.
The sampling light pulse is obtained as an output of an optical sampling gate 111 by driving the short pulse light source 112 with a sampling frequency signal from a sampling frequency generator 113. The sampling frequency generator 113 is made to output, as a sampling frequency signal, a frequency signal (f0/N+Δf) obtained by N-dividing down a clock signal (repetition frequency f0) synchronized with light under measurement and further frequency-shifting it by a frequency Δf for sweep. The repetition frequency represents the inverse number of a repetition period of an optical pulse train.
In addition, the optical sampling gate 111 multiplexes this sampling light pulse and the light under measurement and introduces the multiplexed light into the nonlinear medium of the optical sampling gate 111, thereby generating a nonlinear effect. This nonlinear effect provides an optical signal having an intensity correlation with respect to the light under measurement and the sampling light pulse, i.e., an intensity correlation signal light (wavelength: λc, repetition frequency: f0/N+Δf). This intensity correlation signal light is extracted through an optical filter and outputted from the optical sampling gate 111.
The intensity correlation signal light outputted from the optical sampling gate 111 is converted into an electric correlation signal in a photoelectric converter 114 and, usually, after the conversion from an analog signal into a digital signal through the use of a sampling frequency signal, it is inputted to a vertical axis signal port of a waveform indicator 115 as an intensity correlation signal converted into an electric signal. In the waveform indicator 115, with respect to the intensity correlation signal inputted to the vertical axis signal port, a waveform of a light under measurement is displayed in a manner such that a sweep signal with a frequency Δf to be used in the sampling frequency generator 113 is inputted to a horizontal axis signal port for triggering.
Since the time resolution of the optical sampling shown in FIG. 24 depends principally upon a pulse width of a sampling light pulse, when a sampling light pulse with a short pulse width is prepared, the optical sampling having a high time resolution is realizable. On the other hand, the intensity of a signal light corresponding to the vertical axis depends upon the magnitude of an intensity correlation signal light. This intensity correlation signal light develops in a nonlinear medium, and as this nonlinear medium, a potassium titanyl phosphate (KTiOPO4: KTP) crystal, a periodically polled LiNbO3 (PPLN) crystal or the like are available.
However, since the intensity correlation signal light developing within the aforesaid crystals depends upon the polarization states of the incident light under measurement and the sampling light pulse, difficulty is experienced in measuring a signal light waveform in an arbitrary polarization state.
For this reason, in the non-patent document 1 (N. Yamada et al., “Polarization-insensitive optical sampling system using two KTP crystals,” IEEE Photonics technology letters, vol. 16, no. 1, pp. 215-217, 2004), as shown in FIG. 25, for solving the polarization dependency of the aforesaid intensity correlation signal light and for measuring a signal light waveform in an arbitrary polarization state, there is disclosed a polarization diversity arrangement made to separate an optical pulse into two polarization components of a TE polarization component and a TM polarization component through the use of a polarization beam splitter (PBS) 125 for generating intensity correlation signal lights of the polarization components in two nonlinear mediums. In FIG. 25, reference numeral 120 represents a pulse light source for generating a sampling light pulse, numerals 121 and 122 designate oscillators, numeral 123 depicts a mode locked fiber laser (MLFL), numerals 124 and 127 denote half wave plates (HWP), numerals 126, 129 and 131 indicate mirrors, numerals 128 and 130 show KTPs, numeral 132 signifies an Si-APD (Si-Avalanche Photo Diode), and numeral 133 represents an analog/digital converter, and numeral 134 designates a computer.
As a well-known technique for the removal of the polarization dependency, there has also been known a technique disclosed in the non-patent document 2 (S. Watanabe, et al., “Novel Fiber Kerr-Switch with Parametric Gain: Demonstration of Optical Demultiplexing and Sampling up to 640 Gb/s,” Post deadline session, Th4. 1.6, 30th European Conference on Optical Communication, Sep. 5-9, 2004 Stockholm, Sweden). According to this technique, for the generation of an intensity correlation signal light, it is desirable that the polarization state of each of the light under measurement and the sampling light pulse is a linear polarization and the directions of the respective linear polarizations are different by 40 to 50 degrees from each other. That is, with reference to the linear polarization state of the light under measurement, it is desirable that the linear polarization of the sampling light pulse is inclined by 40 to 50 degrees. Moreover, even if each of the polarization states of the light under measurement and the sampling light pulse is not a linear polarization, there is no problem in the case of a polarization state capable of generating ameasurable intensity correlation signal light. For realizing this, each of the polarization states of the light under measurement and the sampling light pulse, incident on an optical fiber forming a nonlinear medium, is set through the use of a polarization controller. With this technique, there is a condition that the optimum polarization state for the generation of an intensity correlation signal light is not always realizable due to a variation of the polarization state within the optical fiber. In addition, in a case in which each of the polarization states of the light under measurement and the sampling light pulse is set in a manual fashion, it needs long time to set the optimum polarization states of the light under measurement and the sampling light. And not only that, in the case of the manual setting, difficulty is encountered in securing the accuracy and reproducibility. In particular, in a case in which a plurality of lights under measurement is observed simultaneously, there is a need not only to prepare required members corresponding in number to the lights under measurement to be observed at the same time but also to individually set the polarization states of the lights under measurement and the sampling light pulses accordingly, which requires an extremely long time for the initial setting.
The technique described in the aforesaid non-patent document 1 requires two KTPs, each of which is a relatively expensive material.
Moreover, the technique described in the non-patent document 2 does not have a function to monitor an intensity correlation signal light for controlling the polarization state of a light under measurement on the basis of the monitor result, and it does not disclose an arrangement for smoothly observing an actual waveform of the light under measurement regardless of the polarization state of the light under measurement.